Conventional wisdom, and even well-reasoned theoretical mechanisms, suggests that the chronic use of psychoactive substances would impair cognitive functioning of individuals. This article summarizes the research literature with regard to specific drugs of abuse. Undoubtedly, acute intoxication and immediate and protracted withdrawal produce transient alterations of cognitions that can persist for weeks to months. Some subtle residual effects remain for up to 1 year for certain drugs. Evidence of irreversible effects is less clear. Even subtle lingering effects can impact treatment efforts, yet they often go undetected or unaddressed.